Combination intrusion detection devices are well-known in the art. A typical combination is the use of a passive infrared intrusion detection device along with a microwave intrusion detection device. The output of the two sensors are supplied to an AND gate. If both of the sensors detect the presence of an intruder, then an alarm is triggered.
The combination of the electrical outputs of two independent sensing subsystems with each subsystem responding to different stimuli in a complementary manner significantly reduces the possibility of false alarms. This reduction of false alarms more than offsets the higher costs in the manufacturing of these combination intrusion detection devices.
One drawback of a combination dual sensing device is that if one of the sensors or subsystems fails to operate properly, the integrity of the entire system is lost. This is because once a subsystem or the sensor thereof has failed (assuming that it fails in the open mode; i.e., the failed sensor/subsystem never detects the presence of an intruder), and since the entire system is dependent upon the presence of a signal on both of the sensor subsystems, the failure of one sensor subsystem fails the entire system.
There are many possible causes of failure of a sensor or its subsystem. One possible failure of a sensor or its subsystem is the failure in the electrical circuitry. A second possible source of sensor failure is if the sensor is not installed properly. In order for the entire intrusion detection system to function properly, both sensor subsystems must be directed at the same volume or space location. Both sensors must detect the presence of an intruder in the same or proximate location. Thus, there must be overlapping of the area or space of detection of the two sensors. If the two sensor subsystems are not aligned properly and are not directed towards the same space or volume location, the non-overlapping field will result in the entire system never producing alarm. This is because an intruder will always be detected by only a single sensor subsystem. Another source of failure is due to tampering. If a would-be intruder has masked or disabled one sensor subsystem, there again the disablement of that sensor subsystem would have disabled the entire system.
Thus, it is highly desirable in an intrusion detection system of the dual sensor subsystem type to be able to detect any internal electrical malfunction of any one of the sensor subsystems, or to detect any physical tampering of any one of the sensor subsystems, or to detect any masking of the normal fields of use of any of the sensor subsystems or to detect the improper installation which results in substantially different fields of view of each sensor subsystem. Any of these conditions may be termed collectively as a "fault condition".